Removes VRTs on-exit for asset mode

[goergen95]

• raster cell values a explicitly read into memory in asset mode
• VRT files are removed automatically on-exit of .read_raster() to avoid writing too many files on disk when in asset mode

[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.16%. Comparing base (157a843) to head (61bdb82).

Files	Patch %	Lines
R/spatial-utils.R	92.30%	2 Missing ⚠️
R/calc_indicators.R	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
+ Coverage   85.14%   85.16%   +0.01%     
==========================================
  Files          50       50              
  Lines        2269     2272       +3     
==========================================
+ Hits         1932     1935       +3     
  Misses        337      337              

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